243 research outputs found
The mass and radius of the M dwarf companion to GD 448
We present spectroscopy and photometry of GD 448, a detached white dwarf - M
dwarf binary with a period of 2.47h. We find that the NaI 8200A feature is
composed of narrow emission lines due to irradiation of the M dwarf by the
white dwarf within broad absorption lines that are essentially unaffected by
heating. Combined with an improved spectroscopic orbit and gravitational red
shift measurement from spectra of the H-alpha line, we are able to derive
masses for the white dwarf and M dwarf directly (0.41 +/- 0.01 solar masses and
0.096 +/- 0.004 solar masses, respectively). We use a simple model of the CaII
emission lines to establish the radius of the M dwarf assuming the emission
from its surface to be proportional to the incident flux per unit area from the
white dwarf. The radius derived is 0.125 +/- 0.020 solar radii. The M dwarf
appears to be a normal main-sequence star in terms of its mass and radius and
is less than half the size of its Roche lobe. The thermal timescale of the M
dwarf is much longer than the cooling age of the white dwarf so we conclude
that the M dwarf was unaffected by the common-envelope phase. The anomalous
width of the H-alpha emission from the M dwarf remains to be explained, but the
strengh of the line may be due to X-ray heating of the M dwarf due to accretion
onto the white dwarf from the M dwarf wind.Comment: 8 pages, 8 figure
Normal modes of Proca fields in AdS spacetime
A normal mode analysis for Proca fields in the anti-de Sitter (AdS) spacetime
is given. It is found that the equations for the Proca field can be decoupled
analytically. This is performed by changing the basis of the vector spherical
harmonics (VSH) decomposition. The normal modes and the normal mode frequencies
of the Proca equation in the AdS spacetime are then analytically determined. It
is also shown that the Maxwell field can be recovered by taking the massless
limit of the Proca field with care so that the nonphysical gauge modes are
eliminated.Comment: 6 page
Two Distinct Cdc2 Pools Regulate Cell Cycle Progression and the DNA Damage Response in the Fission Yeast S.pombe
The activity of Cdc2 (CDK1) kinase, which coordinates cell cycle progression and DNA break repair, is blocked upon its phosphorylation at tyrosine 15 (Y15) by Wee1 kinase in the presence of DNA damage. How Cdc2 can support DNA repair whilst being inactivated by the DNA damage checkpoint remains to be explained. Human CDK1 is phosphorylated by Myt1 kinase at threonine 14 (T14) close to its ATP binding site before being modified at threonine 161 (T167Sp) in its T-loop by the CDK-activating kinase (CAK). While modification of T161 promotes association with the cyclin partner, phosphorylation of T14 inhibits the CDK1-cyclin complex. This inhibition is further enforced by the modification of Y15 by Wee1 in the presence of DNA lesions. In S.pombe, the dominant inhibition of Cdc2 is provided by the phosphorylation of Y15 and only a small amount of Cdc2 is modified at T14 when cells are in S phase. Unlike human cells, both inhibitory modifications are executed by Wee1. Using the novel IEFPT technology, which combines isoelectric focusing (IEF) with Phos-tag SDS electrophoresis (PT), we report here that S.pombe Cdc2 kinase exists in seven forms. While five forms are phosphorylated, two species are not. Four phospho-forms associate with cyclin B (Cdc13) of which only two are modified at Y15 by Wee1. Interestingly, only one Y15-modified species carries also the T14 modification. The fifth phospho-form has a low affinity for cyclin B and is neither Y15 nor T14 modified. The two unphosphorylated forms may contribute directly to the DNA damage response as only they associate with the DNA damage checkpoint kinase Chk1. Interestingly, cyclin B is also present in the unphosphorylated pool. We also show that the G146D mutation in Cdc2.1w, which renders Cdc2 insensitive to Wee1 inhibition, is aberrantly modified in a Wee1-dependent manner. In conclusion, our work adds support to the idea that two distinct Cdc2 pools regulate cell cycle progression and the response to DNA damage
A photometric and spectroscopic study of NSVS 14256825: the second sdOB+dM eclipsing binary
We present an analysis of UBVRIJH photometry and
phase-resolved optical spectroscopy of NSVS 14256825, an HW Vir type binary.
The members of this class consist of a hot subdwarf and a main-sequence
low-mass star in a close orbit ( d). Using the
primary-eclipse timings, we refine the ephemeris for the system, which has an
orbital period of 0.11037 d. From the spectroscopic data analysis, we derive
the effective temperature, K, the surface gravity, , and the helium abundance, , for the hot component. Simultaneously modelling the
photometric and spectroscopic data using the Wilson-Devinney code, we obtain
the geometrical and physical parameters of NSVS 14256825. Using the fitted
orbital inclination and mass ratio (i = 82\fdg5\pm0\fdg3 and , respectively), the components of the system have , , , and . From its spectral
characteristics, the hot star is classified as an sdOB star.Comment: 8 pages, 7 figures, accepted for publication in MNRA
A Search for Nitrogen-Enhanced Metal-Poor Stars
Theoretical models of very metal-poor intermediate-mass Asymptotic Giant
Branch (AGB) stars predict a large overabundance of primary nitrogen. The very
metal-poor, carbon-enhanced, s-process-rich stars, which are thought to be the
polluted companions of now-extinct AGB stars, provide direct tests of the
predictions of these models. Recent studies of the carbon and nitrogen
abundances in metal-poor stars have focused on the most carbon-rich stars,
leading to a potential selection bias against stars that have been polluted by
AGB stars that produced large amounts of nitrogen, and hence have small [C/N]
ratios. We call these stars Nitrogen-Enhanced Metal-Poor (NEMP) stars, and
define them as having [N/Fe] > +0.5 and [C/N] < -0.5. In this paper, we report
on the [C/N] abundances of a sample of 21 carbon-enhanced stars, all but three
of which have [C/Fe] < +2.0. If NEMP stars were made as easily as
Carbon-Enhanced Metal-Poor (CEMP) stars, then we expected to find between two
and seven NEMP stars. Instead, we found no NEMP stars in our sample. Therefore,
this observational bias is not an important contributor to the apparent dearth
of N-rich stars. Our [C/N] values are in the same range as values reported
previously in the literature (-0.5 to +2.0), and all stars are in disagreement
with the predicted [C/N] ratios for both low-mass and high-mass AGB stars. We
suggest that the decrease in [C/N] from the low-mass AGB models is due to
enhanced extra-mixing, while the lack of NEMP stars may be caused by
unfavorable mass ratios in binaries or the difficulty of mass transfer in
binary systems with large mass ratios.Comment: 14 pages, 7 figures, to be published in Ap
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
Faint Blue Objects in the Hubble Deep Field South Revealed: White Dwarfs, Subdwarfs, and Quasars
We explore the nature of the faint blue objects in the Hubble Deep Field
South. We have derived proper motions for the point sources in the Hubble Deep
Field South using a 3 year baseline. Combining our proper motion measurements
with spectral energy distribution fitting enabled us to identify 4 quasars and
42 stars, including 3 white dwarf candidates. Two of these white dwarf
candidates, HDFS 1444 and 895, are found to display significant proper motion,
21.1 7.9 mas/yr and 34.9 8.0 mas/yr, and are consistent with being
thick disk or halo white dwarfs located at ~2 kpc. The other faint blue objects
analyzed by Mendez & Minniti do not show any significant proper motion and are
inconsistent with being halo white dwarfs; they do not contribute to the
Galactic dark matter. The observed population of stars and white dwarfs is
consistent with standard Galactic models.Comment: ApJ accepted, 39 pages, 13 figure
Four-colour photometry of eclipsing binaries. XL, uvby light curves for the B-type systems DW Carinae, BF Centauri, AC Velorum, and NSV 5783
Aims. In order to increase the limited number of B-stars with accurately known dimensions, and also the number of well studied eclipsing binaries in open clusters, we have undertaken observations and studies of four southern double-lined eclipsing B-type binaries; DWCar, BF Cen, ACVel, and NSV 5783.
Methods. Complete uvby light curves were observed between January 1982 and April 1991 at the Danish 0.5 m telescope at ESO La Silla, since 1985 known as the Strömgren Automatic Telescope (SAT). Standard indices for the systems and the comparison stars,as well as additional minima observations for ACVel, have been obtained later at SAT. For DWCar and ACVel, high-resolution spectra for definitive spectroscopic orbits have also been obtained; they are presented as part of the detailed analyses of these systems.
A few spectra of NSV 5783 are included in the present paper.
Results. For all four systems, the first modern accurate light curves have been established. DWCar is a detached system consisting of two nearly identical components. It is member of the young open cluster Cr228. A detailed analysis, based on the new light curves and 29 high-resolution spectra, is published separately. BFCen is semidetached and is member of NGC 3766. Modern spectra are
needed for a detailed study. ACVel is a detached system with at least one more star. A full analysis, based on the new light curves and 18 high-resolution spectra, is published separately. NSV 5783 is discovered to be an eclipsing binary consisting of two well-detached
components in an 11-day period eccentric (e = 0.18) orbit. Secondary eclipse is practically total. From the light curves and a few high-resolution spectra, accurate photometric elements and preliminary absolute dimensions have been determined. The quite similar components have masses of about 5 M and radii of about 3.5 R, and they seem to have evolved just slightly off the ZAMS. The
measured rotational velocities (â150 km sâ1) are about 6 times those corresponding to pseudosynchronization
Massive runaway stars in the Small Magellanic Cloud
Using archival Spitzer Space Telescope data, we identified for the first time
a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the
detection of their bow shocks. The geometry of detected bow shocks allows us to
infer the direction of motion of the associated stars and to determine their
possible parent clusters and associations. One of the identified runaway stars,
AzV 471, was already known as a high-velocity star on the basis of its high
peculiar radial velocity, which is offset by ~40 km/s from the local systemic
velocity. We discuss implications of our findings for the problem of the origin
of field OB stars. Several of the bow shock-producing stars are found in the
confines of associations, suggesting that these may be "alien" stars
contributing to the age spread observed for some young stellar systems. We also
report the discovery of a kidney-shaped nebula attached to the early WN-type
star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure
created owing to the interaction between the stellar wind and the ambient
interstellar medium.Comment: Accepted by A&
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